In a typical aircraft propulsion system it is known to use liner panels to isolate the hot exhaust gas stream from the exhaust casing and to thereby form a guided path for the cooling air flow. To prevent the hot gas stream from overheating the liner panel it is known to incorporate holes in the liner panel, through which cooling air is flowed. This creates a barrier of cool air between the hot exhaust gas stream and the exhaust casing.
This cooling air is typically directed at an angle to the liner panel in order to increase the cooling efficiency, to minimise entry losses, to improve the mixing between the exhaust gas stream and the cooling air flow. This angle may be up to 70 degrees to the exhaust liner surface normal.
These cooling holes are typically formed by laser or electron beam due to the large quantity of holes that must be formed. Furthermore the use of conventional machining methods such as, for example drilling or milling, would be impractical as the cutting tool would skid as soon as it touched down on the surface of the panel.
While a drill guide could be used for drill location it is likely that the drill would be susceptible to breakage at such shallow angles to the substrate surface. Furthermore the use of end mills would be time-consuming and expensive.